1. Field of the Invention
The present invention relates to an objective lens for converging the light emitted from a light source onto the information recording surface of an optical disk, and also relates to an optical head and an optical disk device for optically recording/reproducing information onto/from an optical disk using the objective lens.
2. Description of the Related Art
An objective lens used for an optical head is designed in view of the predetermined base material thickness of an optical disk. Thus, when an optical disk having a different base material thickness from the predetermined thickness is installed, a spherical aberration is caused, so that the convergence performance is deteriorated and it becomes difficult to precisely record/reproduce information onto/from the optical disk. All of conventional optical disks, including a so-called "compact disk (CD)", i.e., a read-only disk for music replay, a video disk and a magneto-optical disk for data storage, have a uniform base material thickness of about 1.2 mm. Therefore, it has heretofore been possible to record/reproduce information from optical disks of various types by using a single optical head.
On the other hand, a digital video disk (DVD), the specifications of which have recently been unified, uses an objective lens having an increased numerical aperture (NA) in order to realize a high density. If the numerical aperture is increased, then the optical resolution of an optical disk is improved. As a result, the width of a frequency band on which the recording/reproducing operations are enabled can be expanded. However, if an optical disk to be installed has a tilt, then a coma is adversely increased. In general, an optical disk is tilted to a certain degree against an objective lens, because the optical disk itself has a certain deflection and some inclination is almost always involved when the optical disk is installed into an optical disk device. Consequently, a kind of aberration called "coma" is generated in a converged light spot. The coma disadvantageously prevents the convergence performance from being improved even when the numerical aperture is increased.
Thus, the base material thickness of a DVD (it is noted that the "base material thickness" of a DVD corresponds to the thickness of one of a pair of bonded substrates, unlike the cases of conventional optical disks) is reduced to about 0.6 mm such that the coma is not increased even when the numerical aperture of an objective lens is increased. However, reducing the base material thickness of an optical disk means changing the predetermined base material thickness for an objective lens for recording/reproducing information onto/from the optical disk. As a result, the objective lens for the changed base material thickness can no longer be used for recording/reproducing information onto/from conventional optical disks (i.e., CDs, magneto-optical disks for data storage, etc.), i.e., the objective lens is no longer compatible with the conventional optical disks.
In order to solve such a problem, a device using two optical heads such as that shown in FIG. 12 is proposed The device shown in FIG. 12 includes two optical heads 70 and 83. The optical head 70 is used for recording/reproducing information onto/from an optical disk 10 having a base material thickness of about 0.6 mm, while the optical head 83 is used for recording/reproducing information onto/from an optical disk 11 having a base material thickness of about 1.2 mm. It is noted that, in FIG. 12, the left half portion of the optical disk 10 having the base material thickness of about 0.6 mm and the right half portion of the optical disk 11 having the base material thickness of about 1.2 mm are selectively illustrated.
In the optical head 70, the radiated light having a wavelength of about 650 nm which has been emitted from a semiconductor laser device 71 is condensed by a condenser lens 72 to be transformed into a luminous flux 73 of substantially parallel light beams. The luminous flux 73 is p-polarized light, which is incident onto and transmitted through a polarization team splitter 74 and transformed by a quarter-wave plate 75 into substantially circularly polarized light. The circularly polarized light is reflected by a reflective mirror 76 to be incident onto an objective lens 77. The luminous flux 73 transmitted through the objective lens 77 is converged onto the information recording surface of the optical disk 10 having the base material thickness of about 0.6 mm, thereby forming a light spot 78 thereon.
The luminous flux reflected by the optical disk 10 passes through the objective lens 77, the reflective mirror 76 and the quarter-wave plate 75 again to be incident onto the polarization beam splitter 74. Since the reflected luminous flux is transformed by the quarter-wave plate 75 into s-polarized light, the s-polarized light is reflected by the polarization beam splitter 74, passed through a converging lens 79 and a cylindrical lens 80, and is received by a photcodetector 81. The photodetector 81 photoelectrically converts the received reflected luminous flux to form a reproduced signal, forms a focusing control signal in accordance with an astigmatism method, forms a tracking control signal in accordance with a phase difference method and a push-pull method, and then outputs these signals.
An objective lens driver 82 drives the objective lens 77 in the focusing direction and the tracking direction, thereby making the light spot 78 follow the tracks on the surface of the recording medium onto/from which the information is recorded/reproduced.
On the other hand, in the optical head 83, the radiated light having a wavelength of about 780 nm which has been emitted from a semiconductor laser device 84 is condensed by a condenser lens 85 to be transformed into a luminous flux 86 of substantially parallel light beams. The luminous flux 86 is p-polarized light, which is incident onto and transmitted through a polarization beam splitter 87 and transformed by a quarter-wave plate 88 into substantially circularly polarized light. The circularly polarized light is reflected by a reflective mirror 89 to be incident onto an objective lens 90. The luminous flux 86 transmitted through the objective lens 90 is converged onto the information recording surface of the optical disk 11 having the base material thickness of about 1.2 mm, thereby forming a light spot 91 thereon.
The luminous flux reflected by the optical disk 11 passes through the objective lens 90, the reflective mirror 89 and the quarter-wave plate 88 again to be incident onto the polarization beam splitter 87. Since the reflected luminous flux is transformed by the quarter-wave plate 88 into s-polarized light, the s-polarized light is reflected by the polarization beam splitter 87, passed through a converging lens 92 and a cylindrical lens 93, and received by a photodetector 94. The photodetector 94 photoelectrically converts the received reflected luminous flux to form a reproduced signal, forms a focusing control signal in accordance with an astigmatism method, forms a tracking control signal in accordance with a phase difference method and a push-pull method, and then outputs these signals.
An objective lens driver 95 drives the objective lens 90 in the focusing direction and the tracking direction, thereby making the light spot 91 follow the tracks on the surface of the recording medium onto/from which the information is recorded/reproduced.
In the above-described arrangement, in the case of recording/reproducing information onto/from the optical disk 11 having a base material thickness of about 1.2 mm such as a CD, the optical head 83 is operated and controlled such that the light spot 91 is formed on the information recording surface of the optical disk 11. On the other hand, in the case of recording/reproducing information onto/from the optical disk 10 having a base material thickness of about 0.6 mm such as a DVD, the optical head 70 is operated and controlled such that the light spot 78 is formed on the information recording surface of the optical disk 10. In this way, information can be recorded/reproduced onto/from both the optical disks 10 and 11 having different base material thicknesses.
However, in the above-described conventional arrangement, since two optical heads are used, two optical systems including optical members, photodetectors, objective lens drivers, focusing drivers and tracking drivers are required, so that the necessary costs are doubled.
Furthermore, though a distance S between the light spots 78 and 91 is constant, the angle .theta. formed between the line linking the light spots 78 and 91 and an information track 95 on the inner periphery of the optical disk is different from the angle .theta. formed between the line linking the light spots 78 and 91 and an information track 95 on the outer periphery of the optical disk. When the angles .theta. are variable, the diffraction patterns of the information tracks 95 which are included in the reflected luminous flux rotate, thereby varying the levels of the tracking signals and deteriorating the quality of the tracking signals.
Moreover, when an optical disk is installed within a cartridge, the two objectives lens drivers 82 and 95 are required to be disposed within an opening 96 of the cartridge. Thus, since the objective lens drivers 82 and 95 must be downsized, the forces of the objective lens drivers 82 and 95 for driving the objective lenses 77 and 90 are decreased, and it becomes difficult to reproduce information from an optical disk by increasing the rotations per minute of the optical disk.